1. Field of the Invention
The present invention relates to an optical recording method for recording information on an optical recording medium, such as an optical tape or an optical card, and an apparatus for carrying out the same.
2. Description of the Prior Art
A writable optical disk is provided with a preformatted tracks called pregrooves, which are used for tracking control in recording information on the optical disk. Therefore, nearly ideal recording tracks can be formed when ideal preformatted pregrooves are formed.
Optical recording/reproducing apparatus proposed in, for example, Japanese Patent Laid-open (Kokai) Nos. Sho 62-112234 and Sho 63-103440 use a flexible optical recording medium, such as an optical tape. In recording information on an optical tape by these prior art optical recording/reproducing apparatus, the optical tape is wound around the circumference of a rotary drum in a predetermined contact angle, and a light beam emitted by an external laser source is projected on the optical tape by a rotary optical scanning system comprising, as principal components, an image rotator (image rotating optical system), a mirror provided in the rotary drum, and an objective lens, and using a multibeam. As the rotary drum rotates, the light beam is turned so as to scan the optical tape. If the light beam is deviated from the axis of rotation of the rotary drum, the spot of the light beam on the optical recording medium is dislocated when the mirror rotates. This problem can be solved by an image rotator that turns an image about the optical axis.
The image rotator has a rotary member provided with an image rotating prism, such as a dove prism. The image rotating prism rotates the image of a stationary object through twice the angle of rotation of the rotary member. Accordingly, the image rotator rotates in synchronism with the rotary drum at a rotating speed half the rotating speed of the rotary drum. Thus, information is recorded, similarly to recording information for magnetic recording, on diagonal, straight tracks on the optical tape.
Generally, the optical tape used for optical recording is not provided with any pregrooves, i.e., preformatted reference tracks, which are formed on an optical disk. Accordingly, the linearity of the recording tracks on the optical tape is dependent on the mechanical accuracy of the recording apparatus and curved tracks are formed due to errors in the rotary component of the rotary optical scanning system including the image rotator and the rotary drum. If the period of an error causing the tracks to curve is equal to that of rotation of the rotary drum, parallel, curved tracks are formed because each track is formed while the rotary drum turns one full turn, and hence there is little possibility of crosstalk between the adjacent tracks.
However, as mentioned above, since the period of rotation of the image rotator is twice that of rotation of the rotary drum, the error ER causing the tracks to curve varies in a sinusoidal curve having a period twice that of rotation of the rotary drum. Consequently, the effect of the error ER on curving a track in one period TA of rotation of the rotary drum and the effect of the same on curving a track in the subsequent period TB of rotation of the rotary drum are reverse to each other as shown in FIG. 7 and the curvature of a track 2A formed in the period TA and that of a track 2B formed in the subsequent period TB are reverse to each other as shown in FIG. 8. Crosstalk is liable to occur between such adjacent tracks 2A and 2B curving in opposite directions.
The error in the image rotator causing such track curvature is attributable to the unsatisfactory accuracy of the dove prism, which can not be improved even by optical position adjustment with high accuracy. Accordingly, the track pitch must be increased to obviate crosstalk, which, however, reduces recording density.
If such track curvature is characteristic of a recording apparatus, load on the servo system of a recording/reproducing apparatus will be increased in reproducing information recorded on an optical tape recorded by another recording/reproducing apparatus. Such a problem arises also when the period of the error is equal to that of rotation of the rotary drum.
The same problems arises in reproducing information recorded on an optical tape by a recording/reproducing apparatus employing a polygonal rotating mirror, such as a recording/reproducing apparatus disclosed in Japanese Patent Laid-open (Kokai) No. Sho 61-34740 or Sho 62-162261, because positional errors in the reflecting faces of the polygonal rotating mirror and the difference in optical aberration between the reflecting faces of the polygonal rotating mirror are characteristic of the polygonal rotating mirror, and the form of tracks is dependent on the characteristics of the polygonal rotating mirror.